Alterations of the neonatal sensory periphery either by injury of the receptor organ or by elimination of sensory input produces changes in the organization of the cortical recipient areas (Kossut and Hand et al. 1988). We are studying cases of altered organization in the cortical representation of vibrissae in the SI of rats after vibrissa receptor organ damage. This representation-the posteromedial barrel subfield (PMBSF) of SI-contains cellular aggregates of distinctive cylindrical shape that extend through the whole thickness of layer IV. The barrels are separated from each other by relatively narrow acellular septa, and each barrel receives a separate bundle of thalamic afferents. Anatomical data and electrophysiological studies suggest that a one-to-one relationship exists between a vibrissa and a barrel (Van der Loos and Woolsey, 1973; Woolsey and Wann, 1976; Simons and Woolsey, 1979; Welker, 1971). It has also been observed that destruction of the vibrissal follicles (up to the 5th postnatal day) causes a disruption of the normal cytoarchitecture and shrinkage of their respective barrels (Kossut and Hand et al. 1988). Simultaneously, neighboring intact barrels show enlargement. Removal of a row of whisker follicles at birth (up to the 5th day of life) is followed by an altered organization of thalamocortical afferents and changes in the orientation of the dendritic trees of neurons comprising the affected row of barrels (Killackey et al. 1976; Harris and Woolsey, 1980). The earlier the damage to the vibrissa receptor system, the greater is its effect in SI. In order to study these changes in the cortical organization, we are evaluating the 3D pattern of metabolic activity within cortical regions following vibrissa stimulation.